Have you ruled out the possibility of the electrical noise creeping into your signal capture?
I have done similar projects for microphone test and have not faced any issue in the signals. Could you please elaborate more on the connections and electrical setup?
1 Have you ruled out the possibility of the electrical noise creeping into your signal capture?
I tried B&K instrument system （3050A-060）, and used the same microphone (B&K 4966-H-041), B&K doesn't have this problem. I ask NI staff many times ,but they can't slove my problem.
In all of my project(more than 5), it got the same problem.
If you can send a staff go with me , I can so you how it Actual is. And maybe I can borrow a B&K system to Compared.
2 I have done similar projects for microphone test and have not faced any issue in the signals. Could you please elaborate more on the connections and electrical setup?
2.1 First ， I have an sound Lab, with Temperature and humidity control. It uses Frequency converter(Inverter) to Adjust Air circulation fan to achieve the temperature and humidity control of the room.
2.2 The Frequency converter(Inverter) is inside the Distribution cabinet. The Air circulation fan is outside of the Lab(see NI-test)
2.3 Usually, the NI system need to get power from the same Distribution Cabinet. But we find out that the interference can transfer with the power cable. We also use the UPS to support the NI's power. And we find out even we cut off the power cable of the UPS(only USE battery to support NI power), it stll has the interference（Spread through the air）
2.4 The Equipment layout.jpg（Please see the previous reply post） shows four types that I had tried. Only the Lower right One make good performance.
Where are you, my friend?
If you try my way, it need one fan（more than 4Kw) and one Mitsubishi Frequency converter.
In china , it may take 6000~7000 yuan.
Or you can take your NI to somewhere with fan and Frequency converter to try. But you need to make sure you can have a room to keep the background noise lower to 20dB(A).
I am based out of USA and I am not associated with NI. I am a NI product user like you. In this community forum, we try to help each other out based on experiences. I am not trying to reproduce the condition but trying to give inputs to help you find the root cause and a possible solution.
I believe the connection between the Microphone and 4464/4499 are shielded and direct. If this is an IEPE Microphone, there would be no external power source which will rule out an option to add filtering to the power pins of the DUT/Mic.
I also suspect if the monitor is injecting any noise into the gnd of the PXI system. Since you have mentioned that placing the whole set up within the chamber has good performance, if possible, retain only the PC with PXI system inside the chamber and access it over a remote desktop through LAN.